Valve metals are metals on which anodic films can be formed and are referred to as such because of the rectifying characteristic of their anodic oxides. These metals include the vanadium group of elements, namely, the elements of Group Va of the Periodic System, i.e. vanadium, niobium and tantalum. Of these metals, tantalum has been found to be especially desirable in the production of electrolytic condensers and, for the production of such capacitors, it is known to produce tantalum powder by a chemical process.
In particular, it is known to produce tantalum powder by forming a reaction mixture of a double fluoride salt of the valve metal and metallic sodium and/or potassium and to react this mixture in the presence of a doping element to form a melt cake from which the reaction products which are water soluble can be leached to leave the valve metal in the form of a powder or in a form which can be readily broken up to produce the powder.
The product has been found to be especially suitable for use as electrodes for electrolytic condensers and the like.
To produce these electrodes, the powder is compacted to form a "green" body or compact which is thereafter sintered. Upon the sintered electrode a dielectric film is then formed.
For high electrical capacitance, it is important that the valve metal powder have an especially small particle size and hence large surface area. It is known to produce powders in the manner described from, for example, United Kingdom Pat. No. 2,068,924 in which the doping element is phosphorus.
While this process can produce a finely divided product with a large surface area, the method has many disadvantages and even with respect to the particle size and high surface area may not be fully satisfactory.